Present-day eyewear lenses (corrective spectacles, sunglasses, ski/sports eyewear, protective goggles) mist up under adverse conditions. These conditions are, on the one hand, the transition from a cold environment into a warm environment (for example, entering the home from outside in cold temperatures in the winter, or leaving an air-conditioned building in a country with tropical climate conditions), and on the other hand if the eyewear lens makes contact with a source of warm/hot air with high relative humidity. Once the eyewear has misted up, the wearer must typically take off his or her eyewear and either wait until the misting disappears or else remove the misting by wiping with a cloth.
The problem of misting affects optical components in general.
If the misting is looked at under a light microscope, it is observed that the “mist” consists of small droplets of water. These droplets have a diameter of typically 20 μm. The degree of surface occupation by these droplets is approximately 50%, as also predicted by models of kinetic theory. The reason why a misted lens appears milky is that the droplets disrupt the propagation of light.
The reduction in transmission is accompanied by an increase in the scattered light fraction, causing the glass to appear milky. A low contact angle of the water droplets is advantageous, since transmission is then high and the scattered light fraction low.
Given that, under the conditions under which lenses mist up, it is not possible to prevent the condensation of water, one approach at a solution is to treat the surface in such a way that the water drops form a small contact angle with the surface.
Approaches exist for how this can be implemented for optical lenses such as eyewear lenses, for example. These approaches typically involve sprays or liquid-impregnated cloths. The liquids employed in this context are from the group of the hydrophilic surfactants. There is a multiplicity of such products on the market. A phenomenon common to these products is that the anti-mist effect or anti-fog effect is not long-lasting, and it is instead necessary to apply the solution regularly to the surface. There are also approaches which harness physical effects to prevent the development of water drops, examples being venting systems in ski goggles.
There are also solutions for optical lenses such as eyewear lenses which are provided with a hard coating and which ensure a long-lasting anti-mist (anti-fog) effect.
Present-day high-grade optical lenses such as, for example, high-end eyewear lenses are normally provided with an anti-reflection (AR) coating. This coating reduces unwanted and annoying reflections.
There is advantage in managing to modify the surface of AR-coated optical components, such as eyewear lenses, in such a way that a long-lasting anti-mist effect or anti-fog effect is obtained, hence dispensing with the need for regular aftertreatment, with a spray, for example. Particular attention in this context is to be placed on maintaining the optical effect of the anti-reflection coat.